Fast pyrolysis serves to convert carbon-containing source materials such as biomass into a large amount of liquid pyrolysis condensate (pyrolysis oil) as well as a small amount of solid pyrolysis coke and pyrolysis gas. Fast pyrolysis is accomplished, in the context of the aforesaid biomass as source material, with oxygen excluded and within a few seconds, preferably in approximately one second, at approx. 400 to 600° C., preferably about 500° C.; what results is typically a 40 to 80 wt % proportion of biomass oil and only 10 to approx. 30 wt % biomass coke.
Fast pyrolysis (also called flash pyrolysis) is thus a special pyrolysis method in which a particularly large amount of liquid pyrolysis condensate, and little gas and coke, occur. Wood and straw (lignocellulose) in particular can be liquefied to biomass oil at a rate of over 40% to 80% (See E. Henrich, E. Dinjus, D. Maier: Flue gas gasification of liquid pyrolysis products at high pressure—a new concept for biomass gasification, DGMK conference: Biomass energy utilization, Velen, Apr. 22-24, 2002).
The heat requirement for fast pyrolysis is generally met by the combustion of pyrolysis cokes or pyrolysis gases, or a combination of the two. In a well-managed fast pyrolysis system, the calorific value of the pyrolysis gases (depending on the source material) is on the order of the requirement, i.e. in the range of about 10% of the biomass calorific value or even somewhat less. The calorific value of the coke component usually greatly exceeds the requirement (by a factor of more than two), so that only a portion is used.
Allothermic process management is typical for practically all fast pyrolysis methods. In methods that use a heat transfer medium, the latter is caused to circulate, since otherwise it is difficult to accommodate sufficient heat exchange area in the relatively small volume of the pyrolysis reactor. A second fluidized bed combustion reactor that is separated from the pyrolysis reactor on the gas side is usually installed in the heat transfer medium circuit; in this fluidized bed reactor, pyrolysis gas or usually a portion of the pyrolysis coke is combusted with air, and heats the heat transfer medium in controlled fashion directly in the fluidized bed. A number of problems can occur in this context, for example the handling of low-melting-point ash, potential risks of the formation of toxic chlorodioxins and -furans, incomplete CO combustion, etc. Such problems can be avoided by indirect heating of the heat transfer medium from outside in a heat exchanger, albeit at the cost of greater technical complexity because of the limited heat transition coefficients.
In mechanically fluidized reactors such as, for example, screw reactors (e.g. double-screw mixing reactors), a heat transfer bed having particulate solids (grains) of a heat transfer medium is mechanically mixed and transferred. For this, a relatively fine-grained heat transfer medium that can be thoroughly radially mixed is used, because of its large specific surface, for efficient and fast transfer of a quantity of heat to the lignocellulose.
E. Henrich, E. Dinjus, D. Maier: Flue gas gasification of liquid pyrolysis products at high pressure—a new concept for biomass gasification, DGMK conference: Biomass energy utilization, Velen, Apr. 22-24, 2002 describes, by way of example, a facility and method for thermal treatment of materials. The facility encompasses a double-screw reactor having two conveyor screws arranged parallel to one another, rotating codirectionally, and engaging into one another, in which reactor a continuous biomass flow is heated for several seconds, with a continuous flow of sand or coke as heat transfer medium, to a temperature of about 500° C.
Alternatively, EP 1 354 172 B1 describes a single-screw reactor having a rotary oven with a reaction zone, in which zone a conveyor screw for transporting biomass and heat transfer particles is arranged. The heat transfer medium is constituted by balls of metal, ceramic, or silicon carbide (hard material).
When the aforesaid heat transfer particles, also including quartz sand and similar brittle material, are used in the usual fashion, a small amount of very fine abraded material is formed in the heat transfer circuit at each pass, and is discharged and separated along with the pyrolysis coke. The coke, too, is not immediately discharged completely at the first pass, but instead accumulates in the circulating heat transfer medium, especially when mechanical conveying is used, until an equilibrium value is reached. The presence of more coke can undesirably contribute to faster decomposition of the vapors on the catalytically active coke and ash surfaces, and thus reduce organic condensate yields.